(1) Field of the Invention
The present invention relates to the field of Chemical Mechanical Polishing (CMP) of semiconductor surfaces. The present invention specifically relates to methods and apparatus for chemical mechanical polishing of substrates, such as semiconductor substrates, on a rotating polishing pad in the presence of chemically and/or physically abrasive slurry.
(2) Description of the Prior Art
The method of Chemical Mechanical Polishing (CMP) is a widely accepted method for polishing surfaces, such as semiconductor substrates, to a high degree of planarity and uniformity. The CMP process can be applied to planarize semiconductor wafers before semiconductor circuitry is created within the wafers while the same process can also be used to remove surface irregularities or features of high elevation that have been created during the fabrication of the microelectronic circuitry on the semiconductor wafer. Typically a large polishing pad is used during chemical mechanical polishing, this polishing pad is located on a rotating platen. The surface that needs to be polished is positioned against the polishing pad. Both the polishing pad and the surface that needs to be polished are rotated, typically the two surfaces rotate in opposite directions. Chemical slurry, which may include abrasive materials, is maintained on the polishing pad to modify the polishing characteristics of the polishing pad in order to enhance the polishing of the substrate.
In the conventional approach, the wafer is held in a circular carrier, which rotates. The polishing pads are mounted on a polishing platen which has a flat surface and which rotates. The rotating wafer is brought into physical contact with the rotating polishing pad; this action constitutes the Chemical Mechanical Polishing process. Slurry is dispensed onto the polishing pad typically using a peristaltic pump. The excess slurry typically goes to a drain, which means that the conventional CMP process has an open loop slurry flow and therefore uses and dispenses with an excessive amount of slurry that adds significantly to the processing cost. There also is no method for exactly controlling slurry flow.
Since the wafer to be polished, which has a flat surface, and the polishing pad, which in the conventional approach is mounted on a flat polishing table, are both rotating, there exists a velocity differential across the surface of the wafer during the polishing operation. This velocity differential has a negative impact on wafer polishing uniformity and planarity across the die and across the wafer. This limits the application of the conventional CMP approach especially in Shallow Trench Applications, copper damascene, etc., which are involved in sub-quarter micron technology modes.
The use of chemical mechanical polishing to planarize semiconductor substrates has not met with universal acceptance, particularly where the process is used to remove high elevation features created during the fabrication of microelectronic circuitry on the substrate. One primary problem which has limited the used of chemical mechanical polishing in the semiconductor industry is the limited ability to predict, much less control, the rate and uniformity at which the process will remove material from the substrate. As a result, CMP is labor intensive process because the thickness and uniformity of the substrate must be constantly monitored to prevent over-polishing or inconsistent polishing of the substrate surface.
Specifically, applying the CMP process to Intra-Level Dielectric (ILD) and Inter Metal Dielectric (IMD) that are used for the manufacturing of semiconductor wafers, surface imperfections (micro-scratch) typically present a problem. Imperfections caused by micro-scratches in the ILD and IMD can range from 100 to 1000 EA for 200 mm. wafers, where an imperfection typically has a depth from 500 to 900 .ANG. and a width of from 1000 to 3000 .ANG.. As part of the polishing process of the ILD and IMD, a tungsten film is deposited; the surface imperfections will be filled with tungsten during this deposition. For devices within the semiconductor wafer with a dimension of 0.35 um. or larger, an etching process is used where the tungsten that has entered the imperfections within the wafer surface can be removed. For the larger size devices within the semiconductor wafer there is therefore no negative impact on the yield of these devices. For device sizes within the semiconductor wafer of 0.25 um or less, the indicated procedure of etching the tungsten layer is no longer effective. This results in relative large imperfections within the surface of the wafer, large with respect to the size of the semiconductor devices. These imperfections will cause shorts between the metal lines within the devices while the imperfections also have a severe negative impact on device yield and device reliability.
The profile of the polishing pad plays an important role in determining good overall polishing results. The polishing pad can, for instance, be profiled thick at the inner diameter of the polishing pad as compared to the outer diameter of the polishing pad and visa versa. The profile of the polishing pad is typically achieved by trial and error and by adjusting the position of a diamond dresser (see following paragraph). This method of profiling the polishing pad is destructive, time consuming and causes the loss of the polishing pad. Since this measure of the polishing pad profile can only be performed at the end of the useful life of the polishing pad, the wrong profile can only be detected after the polishing pad has served its useful life.
The polishing process is carried out until the surface of the wafer is ground to a highly planar state. During the polishing process, both the wafer surface and the polishing pad become abraded. After numerous wafers have been polished, the polishing pad becomes worn to the point where the efficiency of the polishing process is diminished and the rate of removal of material from the wafer surface is significantly decreased. It is usually at this point that the polishing pad is treated and restored to its initial state so that a high rate of uniform polishing can once again be obtained.
FIG. 1 shows a Prior Art CMP apparatus. A polishing pad 10 is affixed to a circular polishing table 12 that rotates in a direction indicated by arrow 14 at a rate in the order of 1 to 150 RPM. A wafer carrier 16 is used to hold wafer 18 face down against the polishing pad 10. The wafer 18 is held in place by applying a vacuum to the backside of the wafer (not shown). The wafer 18 can also be attached to the wafer carrier 16 by the application of a substrate attachment film (not shown) to the lower surface of the wafer carrier 16. The wafer carrier 16 also rotates as indicated by arrow 20, usually in the same direction as the polishing table 12, at a rate on the order of 1 to 150 RPM. Due to the rotation of the polishing table 12, the wafer 18 traverses a circular polishing path over the polishing pad 10. A force 22 is also applied in the downward vertical direction against wafer 18 and presses the wafer 18 against the polishing pad 10 as it is being polished. The force 22 is typically in the order of 0 to 15 pounds per square inch and is applied by means of a shaft 24 that is attached to the back of wafer carrier 16.
Accordingly, the subject surface (that is the lower surface) of the substrate 18 is polished by the combination of a chemical polishing action of alkali contained in the polishing agent or slurry 26 and a mechanical polishing action by silica contained in the polishing slurry 26. Slurry 26 is provided to the surface of the polishing pad 10 by means of the slurry distribution head 28, slurry supply line 30 is connected to a slurry supply vat (not shown) from where the slurry is provided to the polishing system. The type of slurry used in combination with the method in which the slurry is delivered to the polishing pad have a considerable influence on the effectiveness of the polishing action. Slurry can for instance be delivered by gravity feed or it can be delivered under pressure. Slurry can be of one chemical component or of a mixture of chemical components. It is furthermore of importance to assure that the slurry is evenly distributed over the surface of the polishing pad. The angle and force under which the slurry impacts the surface of the polishing pad are therefore of importance in the design of a chemical mechanical polishing apparatus. Further considerations in this design must be the even and uniform polishing action across the entire surface that is being polished in order to assure uniform planarity and thickness of the surface.
Further to be considered is the effect that irregularities of any kind in the surface of the polishing pad have on the final polishing performance and results of the pad. Microscopic analysis of the surface profile of the polishing pad will reveal unsymmetrical or irregular surface cavities or indentations. The effect of this lack of symmetry of these surface indentures is that the polishing action provided by the polishing pad is dependent on the direction in which the polishing pad crosses the surface that is being polished. This relative direction of motion between the polishing pad and the surface that is being polished reverses with great frequency and unpredictability due to the rotating motion of both the polishing pad and the surface that is be in g polished.
FIG. 2 further illustrates this point. Wafer 35 has a surface 45 that is being polished, polishing pad 39 has a polishing surface 47. The irregularities 37 (in the surface of the wafer) and 49 (in the surface of the polishing pad) are, for purposes of explanation, highly magnified. In the example shown, irregularity 37 (in the surface of the wafer) will strikes edge 43 of the polishing pad indentation 49 in view of the relative motions 51 (of the polishing pad 39) and 53 (of the substrate). It is clear from FIG. 2 that, if the directions of the relative motions 51 and 53 are reversed, the irregularity 37 will strikes edge 41 of the polishing pad indentation 49. Since the angle or incline of edges 41 and 43 are not identical, it follows that the polishing action provided by the polishing pad 39 during the CMP process is not universal. The invention teaches a method that eliminates this irregularity in polishing pad impact during the process of chemical mechanical polishing.
The above impact of irregularities in the surface of the polishing pad is further emphasized by the fact that, using conventional polishing apparatus, the center of the substrate is at a fixed distance from the center of the polishing pad. FIG. 3 shows a plan view of the polishing pad 55 (rotating in direction 63) whereupon a wafer 57 (rotating in direction 65) is being polished. The ratio r1/r2 is fixed and determined by the diameters of the wafer and the polishing pad. This fixed ratio increases the probability that the same irregularities in the surface of the polishing pad (for instance irregularity 59) will come into contact with the same surface unevenness in the surface that is being polished (for instance unevenness 61) whereby this contact is made under the same relative positioning of irregularity with respect to unevenness. This further amplifies the previously highlighted negative impact on polishing effectiveness and quality.
A typical CMP process involves the use of a polishing pad made from a synthetic fabric and polishing slurry which includes pH-balanced chemicals, such as sodium hydroxide, and silicon dioxide particles.
Abrasive interaction between the wafer and the polishing pad is created by the motion of the wafer against the polishing pad. The pH of the polishing slurry controls the chemical reactions, e.g. the oxidation of the chemicals that comprise an insulating layer of the wafer. The size of the silicon dioxide particles controls the physical abrasion of surface of the wafer.
The polishing pad is typically fabricated from a polyurethane (such as non-fibrous polyurethane, cellular polyurethane or molded polyurethane) and/or a polyester-based material. Pads can for instance be specified as being made of a microporous blown polyurethane material having a planar surface and a Shore D hardness of greater than 35 (a hard pad).
U.S. Pat. No. 5,230,184 (Bukhman) shows a distributed polish head assembly that has a plurality of polish pads.
U.S. Pat. No. 5,575,707 (Talieh et al.) teaches a polish pad cluster for polishing a wafer.
U.S. Pat. No. 5,816,891 (Woo) shows a CMP machine with multiple polish pad and carriers.